The removal of particulates from a gas stream has long been a practice in a variety of industrial fields. Conventional means for filtering particulates and the like from gas streams include, but are not limited to, filter bags, filter tubes, filter cartridges and filter panels. These filter elements are typically oriented into a filtration system, often referred to as a filter baghouse, for filtering such particulates. Such filtration systems may be either cleanable or non-cleanable, depending on the requirements of the system operation. Referring to FIG. 1, there is shown one non-limiting example of a conventional filtration system, in this case a pulse jet cleaning system and sequence is shown. Inside hopper 120, the particulate laden gas stream 121 enters the hopper at inlet 122 and passes through filter bag 123. Tube sheet 125 inside hopper 120 prevents the gas stream from bypassing the filter bag. The filter bag 123 is kept open by support cage 126. The gas stream, after passing through the bag and out bag exit 129, exits the clean air compartment at outlet 127. In operation, particulate forms a dust cake 128 on the outside of the filter bag, as shown in the bag on the left of the figure. On cleaning to remove the filter cake, air from pulse pipe 130 enters the bag. This pulse of air 132 expands the bag, loosening the dust cake and thus causing particulate 131 to collect at the bottom of the hopper 120. As seen in the bag on the right of the figure, the pulse jet causes the filter bag to expand.
Activated carbon powders have been used for the capture of toxic contaminants such as mercury in flue gas streams emanating from utility boilers, hazardous or municipal waste incinerators, crematoria and the like. Typically, activated carbon powder is fed, or “injected,” into a flue gas stream upstream of a particulate collection device. One example of such an activated carbon capture system is described in the article entitled Full-Scale Evaluation of Sorbent Injection for Mercury Control on Coal Fired Power Plants by Bustard et al, In Proceedings of Air Quality III: Mercury, Trace Elements and Particulate Matter Conference; Arlington, Va., Sep. 9-12, 2002. This publication teaches that activated carbon powder, can be introduced upstream of a filter bag dust collector (e.g., a baghouse) to adsorb or react with the mercury in the gas stream, then the adsorbed or reacted mercury is collected on the surface of the filter bag or bags.
The problem of the capture and immobilization of gaseous mercury and its compounds has been considered previously. For example, continuous injection of powdered activated carbon (PAC) into the flue-gas train upstream of an electrostatic precipitator or fabric filter has been used to control mercury emissions in the municipal waste combustor industry and has been proposed as a control technology for the coal-fired utility industry in the United States. [John H. Pavlish et. al., “Status Review of Mercury Control Options for Coal-Fired Power Plants”, Fuel Processing Technology, in press (2002), and references therein] Disadvantages of this approach include the need for large volumes of carbon to adsorb mercury to proposed regulatory levels due to the short contact time of the adsorbent carbon in contact with mercury vapor and the low capacity for mercury adsorption by PAC at temperatures above about 130° C. In addition, the requisite C/Hg injection ratios necessitate large volumes of injected carbon that can result in considerable contamination of the fly ash produced in coal-fired utility boilers. Carbon contamination often reduces the commercial value of the ash as an additive for concrete.
Various treatments of PAC with sulfur compounds or elemental iodine to improve equilibrium adsorption capacity or capture efficiency for mercury have been investigated and disclosed. The better performers among these known in the art have been summarized by Pavlish, referenced above. For example, U.S. Pat. No. 3,876,393 discloses the passing of mercury-containing vapors over activated carbon that has been impregnated with sulfuric acid. Unexamined Japanese Patent Application (Kokai) No. 10-109016 (Babcock Hitachi KK) teaches that activated carbon powder, or another component having a large specific surface, treated with a ZnI2 active component can be introduced via a carbon injection system to remove mercury in a flue gas stream.
Disadvantages associated with the use of such systems include expensive injection systems, limited collection efficiencies, particularly at high temperatures (i.e., above 130° C.), and carbon-contaminated fly ash that may require handling as hazardous waste. Initial testing with a Powder River Basin (PRB) ash determined that the presence of even trace amounts of activated carbon in the recovered ash rendered the material unacceptable for use in concrete (Bustard et al).
The use of activated carbon fiber filters for mercury capture is described in Journal of the Air & Waste Management Association, Vol. 50, June 2000, pages 922-929. It is taught that activated carbon fibers can be woven or felted and used in a bag filter configuration where particulate matter and elemental mercury could be captured. However, the adsorptive capacity reported in this work (52.5 micrograms Hg/gram activated carbon) is too low to allow this to be used as a bag filter or fixed bed in place of carbon injection. Furthermore, a bag filter made from activated carbon fibers will trap fly ash particles within the depth of the fiber structure, causing a steep increase in pressure drop over time, and the cleanability of such bags is very limited.
It is known to incorporate catalytic and adsorbent particles into filter elements to react with and/or adsorb components from a gas stream. In U.S. Pat. Nos. 4,220,633 and 4,309,386, to Pirsh, filter bags are coated with a suitable catalyst to facilitate the catalytic reduction process of NOx. In U.S. Pat. No. 5,051,391, to Tomisawa et al., a catalyst filter is disclosed which is characterized in that catalyst particles which are made of metal oxides with a diameter of between 0.01 to 1 um are carried by a filter and/or a catalyst fiber. In U.S. Pat. No. 4,732,879, to Kalinowski et al., a method is described in which porous, preferably catalytically active, metal oxide coatings are applied to relatively non-porous substrates in a fibrous form. In patent DE 3,633,214 A1, to Ranly, catalyst powder is incorporated into multilayered filter bags by inserting the catalyst into the layers of the filter material. Further examples to produce catalytic filter devices include those set forth in JP 8-196830, to Fujita et al., in which a micropowder of an adsorbent, reactant, or the like is supported in a filter layer interior. In JP 4-219124, to Sakanaya et al., a compact, thick, and highly breathable filter cloth is filled with catalyst for the bag filter material in order to prevent catalyst separation. In U.S. Pat. No. 5,620,669, to Plinke et al., the filter comprises composite fibers of expanded polytetrafluoroethylene (ePTFE) having a node and fibril structure, wherein catalyst particles are tethered within the structure. PCT Publication No. PCT/US00/25776, in the name of Waters et al., is directed to filters comprising active particles that are adhered to a porous woven or non-woven substrate by a polymer adhesive, and optionally adjacent or within the substrate is at least one protective microporous layer. However, none of these references discloses or suggests the removal of mercury from a flue gas stream or appropriate chemical composition for effective mercury removal. Moreover, none of these references teaches the removal of particulate and mercury contaminants from a flue gas stream wherein the fly ash particulate is collected at a first location and the mercury is collected downstream of the particulate to minimize or prevent carbon contamination of the particulate fly ash.
An approach to oxidize mercury catalytically to an ionic form that could be removed in a subsequent, downstream wet-flue-gas-desulfurization unit operation (WFGD) was reported by Blythe et al. in “Catalytic Oxidation of Mercury in Flue Gas for Enhanced Removal in Wet FGD Systems”. Promising catalysts were evaluated in packed bed configurations for their abilities to generate a soluble mercury species. The authors anticipated the eventual incorporation of these catalysts into a honeycomb catalytic oxidizer located in the flue-gas train of coal-fired utility boilers immediately after an ESP dust removal unit. Blythe et. al. expected that locating the catalytic oxidizer monolith after the ESP would alleviate high pressure drop caused by fly ash plugging, as had been a concern during their packed bed tests. Although they envisioned the removal of mercury downstream of the point of oxidation and of fly ash removal, their concept requires the installation of a separate unit operation facility with a separate footprint. Furthermore, mercury is recovered in a relatively dilute liquid phase that might require further treatment to concentrate the mercury.
A need clearly exists for an improved filtration system which effectively removes mercury in any oxidation state from flue gases at elevated process temperatures (i.e., >130° C.) without the creation of voluminous byproducts or waste streams. In addition, a need exists for such a system which could be readily retrofit into existing filter systems without significant and expensive modifications to such existing filter systems. A further need exists for a mercury filtration system which provides extended on-line operational capability and less maintenance compared to at least carbon injection systems. Another important need exists for the capability of a single filtration system to provide multi-pollutant control (i.e. particulate, NOx, dioxins, furans, and mercury).
These and other purposes of the present invention will become evident based upon a review of the following specification.